In products consisting of high polymer material such as rubber and plastics for example, there are cases in which reinforcing cords are embedded in the high polymer material in order to increase the strength of such products. Particularly in a pneumatic tire used for heavy duty vehicles and construction vehicles, steel cords are used in many cases as carcass cords, belt cords, and bead cords and the like in order to increase the tire rigidity and to support large tire loads.
As a steel cord of this kind, a cord having what is called a twisted layer structure, which is generally formed by twisting in order a sheath Bi (i=1, 2 . . . ) comprising a plurality of filament f on the outside of a core A formed by twisting three filaments f, as shown in FIG. 8. The reason for advantages of such cord is that the bending rigidity of the cord is uniform and the diameter of the cord is smaller because the filaments f are compactly twisted and the filaments of the core A forms a triangular configuration to stably maintain the filament arrangement with respect to the bending deformation.
However, in the steel cord having such a structure, the filaments f adjoining in each sheath Bi are in close proximity to each other, and permeability of rubber during tire vulcanization is lowered causing these three filaments f to come in close contact in the core A in particular. Therefore, a void H is formed at the center of the core A where rubber does not enter.
As a result of this, the air remaining in the void H expands due to the heat generated in the tire, causing localized distortion. In addition, the moisture, etc. contained in the remaining air and the moisture, etc. entering from a damaged portion of the tire is diffused in the void H, thereby causing tire durability to be deteriorated considerably due to resultant corrosion and damage of the tire.